This invention relates to a hydraulic brake system with slip control. The system is designed for use in motor vehicles equipped with a master cylinder, a hydraulic brake booster connected in front of the master cylinder, at least one static brake circuit connected to the master cylinder and a dynamic brake circuit connected to the pressure chamber of the hydraulic booster. Valves are connected in the brake circuits for the control of the wheel cylinder pressures during slip control. A gasket at the master cylinder piston acts as check valve via which, if required, pressure medium may be supplied from the dynamic brake circuit into the static brake circuit by pressurization of the pedal-close front face of the master cylinder piston. The pedal-close front face of the master cylinder piston is capable of being alternatively connected to the dynamic brake circuit or to an unpressurized return reservoir by means of an electromagnetic switch-over valve.
Such a brake system is described in U.S. Pat. No. 4,415,210. This hydraulic brake system comprises a hydraulic brake booster, with a dynamic brake circuit being connected to the pressured chamber thereof supplying the wheel brakes of the rear axle of a motor vehicle jointly with pressure via an electromagnetically operable 2/2-way valve which normally is open. By means of the booster piston of the hydraulic brake booster, it is possible to actuate a master cylinder coaxially of the hydraulic brake booster and having two master cylinder pistons arranged one behind the other. In this way, two working chambers are formed in the master cylinder which may be pressurized, each of them supplying pressure to a static brake circuit. By means of each static brake circuit it is possible to actuate a wheel brake preferably the front axle of a vehicle.
Associated with each of the master cylinder pistons is a prechamber from which an expansion bore leads into the working chamber, through which communication is established to the pedal-close front face of the master cylinder piston. Both master cylinder pistons are sealed by a gasket acting as a check valve. An electromagnetically operable 2/2-way valve which normally is open is also arranged between each working chamber of the master cylinder and the wheel brake associated with said working chamber.
Both during the brake release and during braking actions below the lock-up limit, an unpressurized return reservoir and the prechambers hydraulically communicate via an electromagnetically operable switch-over valve. As soon as a set lock-up threshold has been reached, the switch-over valve will switch, causing the prechambers and the pedal-close front faces of the master cylinder pistons to become pressurized by the pressure of the dynamic brake circuit. Thus, via the master cylinder gaskets in the working chambers of the master cylinder, it will be possible to replace pressure medium tapped from the static brake circuits for the purpose of a pressure reduction in the front wheel brakes.
Such hydraulic brake systems usually have throttles provided in the electromagnetically operable 2/2-way valves which are normally open. These throttles effect a more steady control behavior and reduce the number of switching actuations of the solenoid valves during a controlled braking action. In this connection it is somewhat disadvantageous that the throttles contained in the solenoid valves are also effective in the initial stage of braking; thus the pressure gradient received by the wheel brakes will be relatively low until a set lock-up threshold. This may have disadvantageous consequences, in particular in such operating conditions in which the driver of the vehicle is forced to generate a relatively high braking pressure in the wheel brakes as quickly as possible.